drop unstable from the testing matrix

several required dependencies have been removed from unstable and
bullseye

Signed-off-by: Mattia Rizzolo <mattia@debian.org>

.gitlab-ci.yml

@@ -15,10 +15,6 @@ before_script:
     paths:
       - cover/
 
-unstable:
-  <<: *test
-  image: debian:unstable
-
 buster-bpo:
   <<: *test
   #image: debian:buster-backports